This invention concerns a conference system of the type that comprises an audio and/or video and/or data bridge to which a multiplicity of terminals can be connected to participate in a conference. The conference bridges or central conference body (each participant is in direct relation with the bridge, and is therefore in relation with the other participants through the bridge) can, for example, be mixing bridges, multiplexing bridges or replicating bridges.
Mixing bridges are sufficiently widespread in nature that it is not necessary to describe their mode of operation here. On the other hand, the same is not true for multiplexing bridges and replicating bridges, which are beginning to be used particularly in the context of IP network conferencing.
The replicating bridge aims to replicate the audio and/or video flows from a correspondent to all the other correspondents who therefore receive several flows to be decoded. The audio flows are often mixed by the receiver before being restored, but are also sometimes each sent over different speakers. The video flows can all be displayed; we are then in a conference context in which all the participants are continuously present.
The multiplexing bridge is based on the same principle. However for a receiver X, instead of simply duplicating the flows of the other participants in order to transmit them to it, this bridge will multiplex the flows of the other participants in order to optimize flow transport by limiting the packet overhead tied to the transport layer. For example, in an IP network, instead of sending N packets corresponding to N remote participants, the multiplexing bridge will send only one packet with the multiplexed data of N remote participants, and will thus achieve a rate gain by factorizing the IP headings, which thus minimizes the packet overhead tied to the IP headers (quite often IP/UDP/RTP).
We can also envision combined mixing-multiplexing bridges or mixing-replicating bridges in order to provide the continuous presence of only M (M<=N) participants out of the N present in the conference.
One of the classic problems is allowing conferences with participants who have different transmission capacities. Indeed, by allowing the interconnection of heterogeneous networks, by offering different access rights, this results in allowing participants with different capacities that can be characterized in terms of input rates and output rates or even using terminals with different calculation capacities, to set up a conference. How, then, can participants with low rates be allowed to participate in a conference without requiring all the conference participants to adopt a mode with minimal or even mediocre quality?
In the case of replicating and/or multiplexing bridges, we realize that the rate of the flow or flows going from the bridge to the participants increases with the number of participants. How, then, can we allow someone to join the conference when the rate received by one of the participants cannot be increased given its network access while retaining the quality of the conference for the participants with network accesses that are not yet saturated?
Another problem is the adaptation of the rate of the flows transmitted to the dynamic variations of the transmission capacities of a participant. The goal being to retain a continuity in the flows transmitted and to avoid cutoffs due to losses that can be caused by saturations of the transmission link.
The current techniques do not allow us to solve these problems completely, but instead propose fall-back solutions.
Thus, when the participants do not all have the same network access capacities, a first solution consists in finding an identical conference mode for all the participants corresponding to the mode that can be accepted by all of them. We then speak of a fall-back mode. The quality of the conference is therefore limited by the participant with the lowest capacity, which is too bad for the participants potentially able to take advantage of a higher level of quality. Furthermore, this also makes it necessary to equip the equipment (terminals and bridges) with several operating modes to allow connections of different qualities. A second solution to this problem can consist in equipping the bridge with elements that play the role of gateways. This time, instead of falling back on the best common mode possible, the conference can take place in a superior mode, but the participants who do not possess the necessary capacities access the bridge via gateways that then perform code conversions. The establishment of such gateways is complex, costly and is a source of quality degradation due to the code conversion and the additional transmission delay resulting from this code conversion. Furthermore, by adding elements to the system, we potentially reduce its reliability.
In the same way, in multiplexing and/or replicating bridges, during a conference where one is already at the capacity limit for at least one of the participants, the only way to admit a new participant (who may also have limited capacities) is to fall back to a lower conference mode. Nevertheless, it is possible to establish more complex techniques. It is possible, for example, to add mixing functionalities or even to limit the number of active participants at the same time using VAD (Vocal Activity Detection) mechanisms and/or speaking authorization mechanisms.
In the case of dynamic degradations of a link, certain techniques consist in reducing the rate transmitted by modifying the behavior of the coders (this is particularly possible with classic video coders). However, taking the example of replicating bridges, a dynamic reduction in the rate of the coder will result in a degradation of this signal for all the participants, even though only one line may present problems. Therefore this solution is not ideal. Furthermore, it obligates the bridge to control the coders of certain terminals based on connections with other terminals.